After performing scavenge, hint OS that data in from spaces are no more neede...

src/platform.h

Index: src/platform.h
===================================================================
--- src/platform.h	(revision 3342)
+++ src/platform.h	(working copy)
@@ -296,6 +296,10 @@
   // Uncommit real memory.  Returns whether the operation succeeded.
   bool Uncommit(void* address, size_t size);
 
+  // Give OS a hint that data of that span could be just discarded
+  // (no need to page it out).  Might be implemented as noop.
+  bool Reset(void* address, size_t size, bool is_executable);
+
  private:
   void* address_;  // Start address of the virtual memory.
   size_t size_;  // Size of the virtual memory.